Document how to use query tracing (#65)

Author: hanahmily

README.md

@@ -135,7 +135,7 @@ client.query(query);
 After response is returned, `elements` can be fetched,
 
 ```java
-StreamQueryResponse resp = client.queryStreams(query);
+StreamQueryResponse resp = client.query(query);
 List<RowEntity> entities = resp.getElements();
 ```
 
@@ -145,6 +145,23 @@ The `StreamQueryResponse`, `RowEntity`, `TagFamily` and `Tag` (i.e. `TagAndValue
 the order of the tag families and containing tags, i.e. indexes of these objects in the List, follow the order specified 
 in the projection condition we've used in the request.
 
+If you want to trace the query, you can use `query.enableTrace()` to get the trace spans.
+
+```java
+// enable trace
+query.enableTrace();
+// send the query request
+client.query(query);
+```
+
+After response is returned, `trace` can be extracted,
+
+```java
+// send the query request
+StreamQueryResponse resp = client.queryStreams(query);
+Trace t = resp.getTrace();
+```
+
 ### Measure
 
 For `Measure`, it is similar to the `Stream`,
@@ -193,6 +210,22 @@ query.offset(1);
 query.orderBy("service_id", Sort.DESC);
 ```
 
+If you want to trace the query, you can use `query.enableTrace()` to get the trace spans.
+
+```java
+// enable trace
+query.enableTrace();
+// send the query request
+client.query(query);
+```
+
+After response is returned, `trace` can be extracted,
+
+```java
+// send the query request
+MeasureQueryResponse resp = client.query(query);
+Trace trace = resp.getTrace();
+```
 ### Criteria
 
 Both `StreamQuery` and `MeausreQuery` support the `criteria` flag to filter data.